Two astronauts will make an unscheduled and hastily choreographed spacewalk early Saturday to try to troubleshoot a leak in a cooling system on the international space station.

The orbital laboratory and the six astronauts aboard are not in peril. But one of its eight solar-powered electrical systems has been shut down because of the leaking ammonia, a coolant that is streaming from an unseen source and sending flakes of ammonia snow drifting into space.

Unplanned spacewalks are very rare, and throw both the space station crew and the flight team back on Earth into overdrive. Astronauts in Houston on Friday plunged into a huge pool containing a mock-up of the station and helped put together a protocol for how their comrades in space can try to tackle the leak.

There’s time pressure because the astronauts need to spot the source of the leak before the coolant completely bleeds out.

In 2013, scientists captured stunning images of the planets, a galaxy, a planetary nebula and more from space.

Caption

Over the past 12 months, scientists have captured stunning images of the planets, a galaxy, a planetary nebula and more from space.

This composite image provided by NASA shows before and after images taken by the Opportunity rover. At left is an image of a patch of ground taken on Dec. 26, 2013. At right is in image taken on Jan. 8, 2014 showing a rock shaped like a jelly doughnut that had not been there before. The space agency said the rover Opportunity likely kicked up the rock into its field of view. Opportunity landed on Mars in 2004 and continues to explore.NASA/AP

Later in the day, he wrote: “What a fun day! This type of event is what the years of training were for. A happy, busy crew, working hard, loving life in space.”

But it’s not an operation without risk. For starters, when the astronauts are outside the station, they will be exposed to micrometeorites, which could puncture their pressure suits. Micrometeorites also are one possible cause of the leak, according to NASA.

The astronauts may also get covered with ammonia, which can be toxic if inhaled. At the end of their spacewalk, the astronauts will perform what’s called a “bakeout,” the orbital version of sunbathing, in which the sunshine warms the spacesuits and causes the ammonia to sublimate away. Then, in the airlock, the astronauts will have to test the air for the presence of ammonia before they reenter the pressurized interior of the station.

During the EVA — extravehicular activity — the two astronauts will replace a unit called a pump flow control subassembly. That’s the suspected source of the leak, though no one has been able to spot it with the cameras on board the station.

Because the leak is not considered an emergency and the station can operate normally even with one power system down, NASA’s approach Saturday will be cautious. “If it doesn’t look right, it’s time to say, we can do this later,” said William Gerstenmaier, NASA’s associate administrator for human spaceflight.

“Spacewalks are not trivial. We don’t do them for fun. We do them when we have to do them,” he said. “You give this team a problem or a challenge, it’s amazing how they rise to the occasion.”

Thursday was supposed to be a rare “light-duty day” for the space station crew, a chance for the Russians on board to mark a national holiday, Victory Day, and also allow three astronauts, including Hadfield, to pack their personal items in anticipation of a return to Earth on Monday. But the astronauts saw small, white flakes drifting away from the station.

Flight engineer Tom Marshburn is still scheduled to fly home Monday in a Russian spacecraft, along with Hadfield and crew mate Roman Romanenko, but first he’ll have to perform the six-hour spacewalk. He’ll be joined on the spacewalk by astronaut Chris Cassidy. Both have performed multiple spacewalks in that part of the station — another reason that NASA decided to go ahead with the repair plan, even though such impromptu EVAs are extremely rare. “The crew is very trained. They’re familiar with the area. It’s almost a perfect setup,” flight director Norm Knight said during a NASA news conference in Houston.

Cassidy and two crew mates, Pavel Vinogradov and Alexander Misurkin, will remain on the station until September. They’ll be joined later this month by three astronauts scheduled to blast off in a Soyuz spacecraft May 28 from Russia’s Baikonur space complex in Kazakhstan.

achenbachj@washpost.com

Joel Achenbach writes on science and politics for the Post's national desk and on the "Achenblog."

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